1. Field of the Invention
The present invention relates to digital image and video processing. More specifically, the present invention relates to methods of detecting and suppressing color-crossing errors for decoded video signals.
2. Discussion of Related Art
Due to advancing semiconductor processing technology, integrated circuits (ICs) have greatly increased in functionality and complexity. With increasing processing and memory capabilities, many formerly analog tasks are being performed digitally. For example, images, audio and even full motion video can now be produced, distributed, and used in digital formats.
FIG. 1 is an illustrative diagram of a portion of interlaced digital video signal 100 most often used in television systems. Interlaced digital video signal 100 comprises a series of individual fields F(0) to F(N). Even fields contain even numbered rows while odd fields contain odd numbered rows. For example if a frame has 400 rows of 640 pixels, the even field would contains rows 2, 4, . . . 400 and the odd field would contains rows 1, 3, 5, . . . 399 of the frame. In general for an interlaced video signal each field is formed at a different time. For example, an interlaced video capture device (e.g. a video camera) captures and stores the odd scan lines of a scene at time T as field F(5), then the video capture device stores the even scan lines of a scene at time T+1 as field F(6). The process continues for each field. Two main interlaced video standards are used. The PAL (Phase Alternating Line) standard, which is used in Europe, displays 50 fields per seconds and the NTSC (National Television System Committee) standard, which is used in the United States, displays 60 fields per seconds. Interlaced video systems were designed when bandwidth limitations precluded progressive (i.e., non-interlaced) video systems with adequate frame rates. Specifically, interlacing two 25 fps fields achieved an effective 50 frame per second frame rate because the phosphors used in television sets would remain “lit” while the second field is drawn.
To ease transmission of video signals, chrominance information and luminance information are combined via modulation into a single composite video signal. Imperfect decoding of composite video signals in either PAL or NTSC format may lead to color-crossing. Specifically, color-crossing error often appears in a video image where the local luminance spatial frequency is near the sub-carrier frequency of the chrominance information. Color-crossing errors occur in both PAL and NTSC video signals.
For example, NTSC video signals typically have a chrominance sub-carrier frequency of 3.58 MHz, i.e., chrominance information is modulated by a sinusoid signal with a frequency equal to 3.58 MHz before transmission. Luminance information may also have components that overlap with the chrominance information near the chrominance sub-carrier frequency. Thus the luminance components near the chrominance sub-carrier frequency cause color-crossing errors, which cannot be cleanly removed. Generally during video decoding a band pass filter at the chrominance sub-carrier frequency is used to obtain the chrominance information. However, the luminance components, which are near the chrominance sub-carrier frequency, are not blocked by the band pass filter. Therefore, the decoded chrominance signal would include “unclean” chrominance information. The color-crossing errors produce rainbow like color blinking in the decoded video image. In PAL video signals, the same color-crossing errors also occur at the PAL chrominance sub-carrier frequency of 4.43 MHz. Color-crossing error can also occur in other encoded video signals.
Conventionally, 3D comb filters have been used to reduce color-crossing errors. Specifically, in NTSC composite video signals the chrominance of corresponding pixels in two consecutive fields of the same type (odd or even) have a phase difference equal to 180 degrees. A 3D comb filter can cancel the miss-included luminance components by a simple subtraction of the video signal values of the two corresponding pixels, when the video image is not changing. However, for PAL composite video, the chrominance of corresponding pixels in two consecutive fields of the same type have only a 90 degree phase difference. Thus, to use 3D comb filters to correct color-crossing errors in decoded PAL composite video signals, four fields must be used.
While 3D comb filters can reduce color-crossing errors, 3D comb filters may also degrade other aspects of video quality. For example, 3D comb filters are very sensitive to noise in composite video signals; therefore, a digital video decoder with a 3D comb filter would have difficulties with weak video signals, which are common in many areas. Furthermore, high quality 3D comb filters are very expensive relative to other components of a video system. Hence, there is a need for a method or system that can efficiently reduce color-crossing errors from decoded composite video signals.